Dielectric layer for discharge lamps and corresponding production method

ABSTRACT

The invention relates to a printing method for applying dielectric layers made of solder glass on strip-shaped metal electrodes of discharge lamps, which are operated by means of pulsed dielectrically inhibited discharge. An aggregate having a higher melting temperature than solder glass, e.g. crystalline or amorphous aluminum oxide or quartz glass powder, is used as printing paste in said method. The typical percentage by weight of the aggregate is between 2 and 30%. Better wetting of the strip-shaped metal electrode is thus obtained.

TECHNICAL FIELD

The present invention relates to dielectric layers for discharge lampswhich are operated by means of dielectric barrier discharge, to aprocess for producing layers of this type and to a discharge lamp havingat least one of these dielectric layers.

In lamps of this type, either the electrodes of one polarity or all theelectrodes, i.e. of both types of polarity, are separated from thedischarge by means of a dielectric layer (“one-sided or two-sideddielectric barrier discharge”). Electrodes of this type are alsoreferred to below as “dielectric electrodes” for short. A dielectriclayer of this type is also referred to as a dielectric “barrier” or“barrier layer”, and the discharge generated with an arrangement of thistype is known as “barrier discharge” (dielectric barrier discharge, e.g.EP-A-0 324 953, page 4).

Dielectric electrodes are produced firstly by the electrodes beingarranged outside the discharge vessel, e.g. on the outer wall, forexample in the form of thin, metal strips which are parallel to oneanother and are of alternating polarity. Discharge lamps of this typeare known, for example, from WO 94/23442 (FIGS. 5a, 5b) and WO 97/04625(FIGS. 1a, 1b).

To protect against contact and/or external influences and to avoidsliding discharges between the electrodes of different polarity, theelectrode strips may advantageously be covered with a thin dielectriclayer, for example with a layer of glass.

Secondly, dielectric electrodes are produced by electrodes which arearranged inside the discharge vessel and are completely covered by adielectric layer. In so-called flat reflectors, the dielectricelectrodes are typically produced in the form of thin, metal stripswhich are arranged on the inner wall of the discharge vessel and, inaddition, either individually—by means of thin, dielectric strips—ortogether—by means of a single, cohesive dielectric layer—are completelycovered with respect to the interior of the discharge vessel. Dischargelamps of this type are known, for example, from EP 0 363 832 (FIG. 3)and German patent application P 197 11 892.5 (FIGS. 3a, 3b).

For the sake of simplicity, the terms “dielectric barrier layers ordielectric protective layers” are combined below under the term“dielectric layers”.

In the present context and in the text which follows, the term“discharge lamp” is intended to mean radiation sources which emit light,i.e. visible electromagnetic radiation, and also ultraviolet (UV) andvacuum ultraviolet (VUV) radiation.

PRIOR ART

One possible option for covering thin, strip-like electrodes with thedielectric layers described in the introduction consists in fusing asuitably sized glass film, if appropriate with the aid of anintermediate layer of soldering glass, onto the electrode strips inquestion.

The drawbacks of this option are firstly the relatively high costs ofsuitable thin glass films, and also their high sensitivity to fracture.These drawbacks have hitherto stood in the way of automated, inexpensiveproduction.

In principle, said layers can be applied more easily and at lower costby means of the screen-printing technique. For this purpose, glasspowder (glass frit)—the screen-printing paste—which is dispersed in asuitable organic solvent—the so-called screen-printing medium—is appliedto the electrodes and to that surface of the discharge vessel whichsurrounds the electrodes with the aid of a so-called squeegee and aresilient screen. The screen is initially arranged at a certain distancefrom the surface. During the application, the squeegee passes over thescreen, so that this screen, together with the printing paste, ispressed onto the surface. In the process, the squeegee fills the meshesin the screen with the printing paste, the squeegee simultaneouslywiping the excess printing paste off the screen. After the squeegee haspassed over the coated meshes, the corresponding meshes are lifted offthe surface again, and the applied printing paste remains on thesurface. After drying, the layer applied is fused, so that ahermetically sealed surface which is as far as possible planar and freeof pores is formed. This is important since the thickness of the layeris a parameter which has a direct effect on the dielectric discharge, onthe one hand, and the high-voltage contact protection, on the otherhand.

However, a drawback is that the surface stress of the fused layer ofsoldering glass prevents complete wetting of the electrodes. Rather, ithas been found that the fused soldering glass draws back from themetallic electrodes over a large area.

PRESENTATION OF THE INVENTION

The object of the present invention is to eliminate the above drawbacksand to provide a dielectric layer which completely covers at least apartial region of one or more electrodes and, in addition, covers atleast the discharge-vessel wall which is immediately adjacent to thispartial region of the electrode. A further aspect of the invention isthat this layer be suitable as a dielectric barrier for a dielectricbarrier discharge, in particular for a pulsed dielectric barrierdischarge.

This object is achieved by the features of claim 1. Further advantageousfeatures are given in the claims which are dependent on claim 1.

A further object is to specify a printing process for applying adielectric layer, in which the printing paste, in the molten state,completely wets at least a partial region of metallic electrodes andalso wets the discharge-vessel wall which is immediately adjacent tothis partial region of the electrode, and consequently, after it hasbeen fired, also completely covers the at least one partial region ofthe electrodes, including the immediately adjacent discharge-vesselwall, with a dielectric layer.

This object is achieved by the features of the process claim. Furtheradvantageous features are given in the claims which depend on thisclaim.

Moreover, protection is claimed for a discharge lamp which has at leastone electrode which is covered with a dielectric layer produced usingthe process according to the invention:

According to the invention, the dielectric layer, which is producedsubstantially from a powder or powder mixture of vitreous substances,additionally contains at least one additive, the melting temperature ofwhich is higher than the melting temperature of the glass powder or theglass powder component with the highest melting temperature.Consequently, the fired layer comprises a vitreous main component inwhich the at least one additive is included in dispersed form, forexample in the form of grains.

If T_(s1) denotes the melting temperature of the glass powder—which istypically approximately 400 to 700° C. —and T_(s2) denotes the meltingtemperature of the additive, the following relationship applies:T_(s2)>T_(s1). It has been found that good results can be achieved withadditives whose melting temperature is at least 100° C. higher than themelting temperature of the glass powder or the glass powder componentwith the highest melting temperature, i.e. the following relationshipapplies: T_(s2)≧100° C.+T_(s1), where the values for T_(s1) and T_(s2)are to be given in ° C.

Suitable additives are in particular powders comprising ceramicsubstances and/or crystalline or amorphous metal oxides, e.g.crystalline or amorphous aluminum oxide powder with a meltingtemperature of more than 2000° C. and/or quartz glass powder with amelting temperature of more than 1400° C. The proportion by weight madeup by the additive or additives is between approximately 2% and 30%,preferably between 5% and 20%. Below the lower limit, the positiveeffect of the at least one additive is no longer sufficient. Above theupper limit, cracks and similar mechanical disruptions to the layeroccur to an unacceptable extent.

The process according to the invention for producing the abovementioneddielectric layer proposes that the abovementioned at least one additivebe admixed with the printing paste containing the glass powder prior tothe actual printing process, advantageously in fine-grained form. As hasalready been mentioned, the proportion by weight made up by the additiveor additives is between approximately 2% and 30%, preferably between 5%and 20%. In this context, it is essential for the effect according tothe invention that the at least one additive be specifically selected insuch a manner that its melting temperature is higher than the firingtemperature required for fusing the glass powder. Otherwise, thestatements which have already been made in connection with theexplanation of the dielectric layer apply in terms of suitableadditives.

A recommended suitable printing process is the standard screen-printingprocess. To produce dielectric layers of relatively great thickness,further printing and melting operations are applied to the previouslayer(s). Since in this case there is no longer any need to cover anyfree electrode surfaces, and consequently there are also no longer anywetting problems, these subsequent layers can also be produced from puresoldering glass powder, i.e. without additive(s).

The printing paste according to the invention, i.e. printing pasteincluding additive(s), can also be used, in a simple, readilyautomatable and consequently inexpensive way, to apply dimensionallystable dielectric layers of any desired dimensions to metallicelectrodes and the surrounding surface of the discharge vessel,specifically with a wetting behavior which is improved compared toprevious pastes.

DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to anexemplary embodiment. In the drawings:

FIG. 1a shows a first sheet of a flat reflector with dielectric layersand electrodes in strip form,

FIG. 1b shows a sectional view, on line AA, through the first sheetshown in FIG. 1a,

FIG. 2 shows a flowchart for the process for applying dielectric layers.

FIGS. 1a, 1 b respectively show diagrammatic views of a plan view and asection on line AA of a first sheet 1 of a flat reflector withelectrodes 2, 3. The first sheet 1 forms part of the discharge vessel ofthe flat reflector, which is completed by a second sheet (not shown),which is parallel to the first sheet, and a frame (not shown). Firstsheet 1 and second sheet are joined in a gastight manner to the frame bymeans of soldering glass (not shown) in such a manner that the interiorof the discharge vessel is of cuboidal design.

The first sheet 1 comprises a base sheet 2 and in each case three anodes3 and cathodes 4 which are in strip form and are made from silversolder, arranged alternately and parallel to one another on the basesheet 2. The anodes 3 are each covered with a dielectric layer 5 of leadborosilicate glass, to which aluminum oxide has been added as additive.FIG. 2 diagrammatically depicts the process for applying the dielectriclayers 5 from FIGS. 1a, 1 b using a flowchart. For this purpose, aprinting screen is used, in which previously all the regions which arenot required to form part of the desired printed image have been coveredby a coating (not shown). After the screen has been placed onto thebaseplate including the electrodes, the printing paste is applied to thescreen. The printing paste comprises 25 g of soldering glass powder(Schott 8465/K6) and 7.5 g of screen-printing medium (Cerdec 80840), towhich 5 g of aluminum oxide powder (Reynolds RC/HP-DBM) have previouslybeen added as additive. Then, a squeegee is passed over the screen.After the screen is removed, the layer which has been applied is driedand then fired at 550° C. The dielectric coating is then finished.

The above example is of purely exemplary nature. Naturally, the processaccording to the invention can also be applied to flat radiators havingmore or fewer than nodes and to other forms of discharge lamps, fortubular discharge lamps.

Further examples relating to the inventive application of a dielectriclayer are listed in the tables below.

TABLE 1 Example 2: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 550° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder  5 α-Al₂O₃ (additive) Reynolds, RC/HP-DBM  5 5% strengthPolyox Union Carbide, WSRN 3000 solution in H₂O  3 H₂O —

TABLE 2 Example 2: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K4 powder  5 γ-Al₂O₃, highly Degussa, disperse (additive)Aluminiumoxid C  5 5% strength Polyox Union Carbide, WSRN 3000 solution 3 H₂O —

TABLE 3 Example 4: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 550° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 1.32 Quartz powder Schott Quarzal, (additive) d₅₀ = 2.25μm 3 5% strength Polyox Union Carbide, WSRN 3000 solution 5.7 H₂O —

TABLE 4 Example 5: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 1.32 γ-Al₂O₃ (additive) Sumitomo, No. 1 5 5% strengthPolyox Union Carbide, WSRN 3000 solution 6 H₂O —

TABLE 5 Example 6: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 1.32 MgO (additive) Produced by applicant 5 5% strengthPolyox Union Carbide, WSRN 3000 solution 8 H₂O —

TABLE 6 Example 7: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 12.5 α-Al₂O₃ (additive) Sumitomo, CAH 5000 1 SiO₂(additive) Wacker, HDK T40 10 5% strength Polyox Union Carbide, WSRN3000 solution 20 0.7% strength Kelco Kelzan solution

TABLE 7 Example 8: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 6.25 α-Al₂O₃ (additive) Sumitomo, CAH 5000 0.5 γ-Al₂O₃,highly Degussa, disperse (additive) Aluminiumoxid C 5 5% strength PolyoxUnion Carbide, WSRN 3000 solution 10 Screen-printing Cerdec 80840 medium

TABLE 8 Example 9: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 6.25 α-Al₂O₃ (additive) Reynolds, RC/HP-DBM 0.5 Alon C(additive) Degussa, Aluminiumoxid C 5 5% strength Polyox Union Carbide,WSRN 3000 solution 12 Screen-printing Cerdec 80840 medium

TABLE 9 Example 10: Application of the above screen- printing paste,drying and subsequent firing of the paste at approximately 600° C.Amount in g Component Company, designation 25 Soldering glass Schott,8465/K6 powder 8.3 α-Al₂O₃ (additive) Reynolds, RC/HP-DBM 5Screen-printing Cerdec 80840 medium 4 H₂O —

What is claimed is:
 1. A dielectric layer produced from a powder orpowder mixture of vitreous substances, the layer additionally containingat least one additive, the melting temperature of which is higher thanthe melting temperature of the glass powder or of the glass powdercomponent with the highest melting temperature, characterized in thatthe proportion by weight formed by the at least one additive is in therange between 5% and 20%, and the layer is suitable for a dischargelamp, which discharge lamp is suitable for operation by means ofdielectric barrier discharge, the discharge lamp having the following: adischarge vessel which at least partially comprises an electricallynonconductive material, and electrodes which are arranged on thedischarge-vessel wall, the dielectric layer completely covering at leastone electrode at least in a partial region, and the dielectric layeradditionally covering at least the discharge-vessel wall which isimmediately adjacent to this partial region of the electrode.
 2. Thelayer as claimed in claim 1, in which the melting temperature of theadditive is at least 100° C. higher than the melting temperature of theglass powder or of the glass powder component with the highest meltingtemperature.
 3. The layer as claimed in claim 1, in which the at leastone additive contains crystalline or amorphous metal oxide, inparticular crystalline or amorphous aluminum oxide.
 4. The layer asclaimed in claim 1, in which the at least one additive contains quartzglass.
 5. A discharge lamp which is suitable for operation by means ofdielectric barrier discharge, having a discharge vessel which at leastpartially comprises an electrically nonconductive material, whichdischarge vessel contains a gas fill in its interior, electrodes whichare arranged on the discharge-vessel wall, at least one electrode beingcompletely covered by means of a dielectric layer at least in a partialregion and the layer additionally covering at least the discharge-vesselwall which is immediately adjacent to this partial region of theelectrode, characterized in that the dielectric layer has all thefeatures of claim
 1. 6. A process for producing a dielectric layer for adischarge lamp which is suitable for operation by means of dielectricbarrier discharge, having a discharge vessel which at least partiallycomprises an electrically nonconductive material, electrodes which arearranged on the discharge-vessel wall, at least one electrode beingcompletely covered by means of the dielectric layer at least in apartial region and the layer additionally covering at least thedischarge-vessel wall which is immediately adjacent to this partialregion of the electrode, for which purpose a printing paste, whichprinting paste contains a powder or powder mixture of vitreoussubstances, at least one additive is added to which printing pastebefore the printing paste is printed onto the electrode(s), is printedand then fused onto the electrode(s), the melting temperature of theadditive being higher than the melting temperature of the glass powderor the glass powder component with the highest melting temperature, andthe proportion by weight made up by the additive or, if appropriate, thesum of the proportions by weight made up by the additives lying in therange between 5% and 20%.
 7. The process as claimed in claim 6, in whichthe melting temperature of the additive is at least 100° C. higher thanthe melting temperature of the glass powder or the glass powdercomponent with the highest melting temperature.
 8. The process asclaimed in claim 6, in which the at least one additive containscrystalline or amorphous metal oxide, in particular crystalline oramorphous aluminum oxide.
 9. The process as claimed in claim 6, in whichthe at least one additive contains quartz glass.
 10. The process asclaimed claim 6, in which the printing is carried out by means of thescreen-printing technique.